The most common answer:
\[ \frac{1}{n} \sum_{i = 1}^{n} (x_i - q_{.5})^2 \]
Other creative answers:
- "modal range": \[ mode_{upper\,quartile} - mode_{lower\,quartile}\]
- meta-variance: take the mean of the variance, sd, IQR, and range
- look at the max height of the density plot